Optimised placeholder placement

[alexreardon]

Right now when you hover over a foreign list the original draggable placeholder remains. This is to avoid the list collapsing. However, I think it would be better to enable the Droppable placeholder for the home list to avoid collapsing but to avoid having a gap in the list.

Full placeholder rules:

• when dragging over a home list - use the Draggable placeholder
• when dragging over no list - use the Draggable placeholder
• when dragging over a foreign list - use the Droppable placeholder in both the home and foreign Droppable.

[alexreardon]

I did a spike of this and it looks quite nice.

[souporserious]

First, thank you for such a great library! I absolutely love the workflow compared to other react sortable solutions. Any idea on when/if this might land? Just started using this library and would love this feature. I'm mainly using this for multiple vertical lists right now so it's a little wonky with the placeholder left over when moving to a foreign list. I'd love to help out with whatever if possible. Please let me know.

[alexreardon]

I hope to look at this as we updated our placeholder logic for #202

[alexreardon]

See 10.1